Study Sheds Light on Butterfly MigrationEvery year, monarch butterflies travel more than a thousand miles from Canada to Mexico. New research might help explain how the monarch's tiny brain handles such a big journey.

Every year, monarch butterflies travel more than a thousand miles from Canada to Mexico. New research might help explain how the monarch's tiny brain handles such a big journey.

STEVE INSKEEP, host:

The annual migration of the monarch butterflies is one of the world's ecological wonders. Every year some monarchs fly more than 1,000 miles from eastern Canada to a tiny forest near Mexico City and then back again. At times they fly more than 10,000 feet above the ground. Now researchers have some new insights into how the monarch's tiny brain helps it navigate. NPR's John Nielsen has the story.

JOHN NIELSEN reporting:

Insects that migrate are typically studied by insect experts known as entomologists, period. But there's nothing typical about the monarch butterfly or its amazing journey south. It gets studied by physicists with an interest in flight and even experts on the human brain. Steven Reppert, a neuroscientist at the University of Massachusetts Medical School, is one of them. He finds the monarch's tiny brain amazing.

Mr. STEVEN REPPERT (Neuroscientist): It's unbelievably cool, and that's why I study it. But I think one must also appreciate that that fundamental knowledge, you never know, down the line may be very important in telling us something about not only how the butterfly brain works but how our brain works.

NIELSEN: Reppert's part of a group of scientists who've been unraveling the monarch's secrets for several years now. Three years ago, he helped prove that migrating monarch use the sun as a compass. Barry Frost(ph), another scientist who studies the brains of humans and of monarchs, says Reppert's discovery raised all sorts of interesting questions. For example...

Mr. BARRY FROST (Scientist): How do they do it on a cloudy day?

NIELSEN: To find out, Reppert and some colleagues put monarchs inside butterfly flight simulators. These are little open barrels, each with a butterfly inside them attached to a tiny thread by a piece of beeswax. The researchers then put the tethered butterflies out under the open sky and watched to see which way the monarchs flew. As expected, they followed the sun when it was up, clouds or no clouds. But when Reppert and his colleagues put a filter on the barrel that blocked out the sun's ultraviolet rays, Reppert says the insects got all confused.

Mr. REPPERT: The butterflies could no longer tell what direction they were going and they essentially flew in circles.

NIELSEN: Reppert says that proves that the butterflies were cuing on ultraviolet rays that are invisible to humans, rays that cut through even heavy clouds. Reppert's team then went into the laboratory to take a close look at the monarch's eyes and brain. What they found were cells in the eye that sensed the presence of this UV light and cells in the brain that were directly linked to the eye cells. The brain cells are the monarch's biological clock, he says. The eye cells are like a compass. And as any good boat pilot will tell you, all you need is a clock and a compass to know which way is south. Reppert says the monarch is a very good pilot.

Mr. REPPERT: So that at the end of the day, the animal actually knows at every sort of minute or every hour how they need to correct for the information they're getting from the sun or the daylight sky in order to maintain this fixed bearing.

NIELSEN: And that's how the monarch butterfly gets to Mexico and back. Now Reppert hopes that this study will help focus attention on the problem unrelated to butterfly neuroscience. That would be the disappearance of its habitat. In Mexico, he says, three-quarters of the forests that are the monarch's wintering grounds have been cut down in recent years. In the United States and Canada, pesticides and development are wiping out butterfly rest stops and the milkweed plant that it depends on. In other words, he says, all the sophisticated navigational gear in the world won't do much good if the monarch butterfly has nothing to eat and nowhere to go.

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